The Long Term Evolution (LTE) communication system is a successor to the Wideband Code Division Multiple Access (WCDMA) and High Speed Downlink Packet Access (HSDPA) systems. In an LTE system, Orthogonal Frequency Division Multiple Access (OFDMA) is used in downlink whereas Single-Carrier Frequency Division Multiple Access (SC-FDMA) is used in uplink. SC-FDMA is closely related to OFDMA and was chosen for uplink to provide a lower peak-to-average power ratio. Such a ratio is not as important in downlink and thus OFDMA is suitable for downlink.
OFDMA is advantageous in that it enables the high data rates that are envisioned for 4G systems. To better appreciate its advantages, consider the case if data is sent in a serial fashion. In a serial data stream, each data symbol must occupy a relatively brief timespan so as to achieve the desired high data rate. For example, if a system is to achieve 100 Mbps and the data is sent serially, each binary data symbol would have a period of just 0.01 μs. If the channel were perfect, such a symbol length would be no problem. But the channel for real-world cellular communication systems is far from perfect: there are lots of buildings, structures, and other obstacles that reflect the signal and introduce multipath. In a multipath channel, various delayed versions of the signal are received in addition to any un-reflected signal. These delayed versions cause inter-symbol interference (ISI) in the receiver that makes serial communication problematic. But OFDMA modulates a high-speed serial data stream onto a parallel stream of various subcarriers. Each subcarrier has a relatively slow data rate, which mitigates the ISI. Moreover, the subcarriers are readily produced through Fourier transform processes. Thus, a handset need have just a single oscillator to produce all the required subcarriers, which eases production costs. Because OFDMA enables robust high-speed and spectrally efficient data transmission at relatively low cost, older technologies such as CDMA are being rapidly phased out in favor of OFDMA systems such as LTE.
Although LTE is advantageous as compared to older technologies, it still faces numerous technical challenges. For example, the desired downlink data rate for LTE is 100 Mbps. Although this is technically possible, it is difficult to achieve in the challenging real-world channel that cellular communications must operate in. For example, if the channel is particularly problematic, the LTE coding rate is reduced accordingly. Thus the LTE goal of 100 Mbps remains largely theoretical in many environments.
Another impediment to high-speed data rates in LTE is the channel bandwidth: although the channel can be made smaller, 20 MHz is the maximum single channel bandwidth under the LTE standard for any given carrier. In LTE, each subcarrier is separated by 15 KHz from adjacent subcarriers. Thus, any given amount of bandwidth can only accommodate so many subcarriers. In LTE the subcarriers are arranged in resource blocks. A 20 MHz channel can thus accommodate only 100 resource blocks. If the channel conditions are such that the subcarriers in each resource block are modulated at a relatively low coding rate, then there is only so much data that can be sent through a 20 MHz channel. In contrast, higher data rates can be achieved with higher coding rates such as 64 QAM. But a handset cannot readily decode 64 QAM unless the channel is of good quality. It is thus often the case that LTE data rate goals cannot be achieved even using the maximum 20 MHz channel bandwidth.
Carrier aggregation allows a communication system such as an LTE system to get around this bandwidth bottleneck. In carrier aggregation, the user equipment may receive (or may transmit) data on multiple carriers. Each carrier is referred to as a component carrier. For example, a base station could use 5 component carriers to provide five 20 MHz channels between it and the handset. Although carrier aggregation thus greatly aids in achieving high data rates, it comes with assorted technical challenges as well. By and large, the data demands for given handset are such that carrier aggregation is not necessary. It is only during periods win which a user equipment is running a data-hungry application such as downloading high-definition video that carrier aggregation becomes necessary. Thus, it is conventional for a user equipment to communicate only on a single carrier with the base station during periods of normal (not excessive) data demands. This single carrier may be designated as the primary component carrier. If the user equipment is to then switch to carrier aggregation operation, the additional secondary component carriers must first be discovered. Each secondary component carrier may also be denoted as an “extension carrier” in that the bandwidth is being extended through carrier aggregation. Alternatively, the secondary component carriers may be denoted as “inter-frequency carriers” in that they are of different frequencies as compared to the primary component carrier.
To better appreciate the problems faced by a handset in discovering the secondary component carrier(s), the discovery required for conventional handoff may first be discussed. If a user equipment (UE) corresponds to a mobile user that is moving away from the serving base station to a neighboring one, that UE should be able to discover this neighboring base station so that a handoff decision can be made. Generally, this handoff decision is based on the received signal quality at the UE. As the UE gets closer to a neighboring base station and farther away from the serving base station, there will be a point when the signals from the neighboring base station are of better quality than those received from the serving base station. When this signal quality difference is deemed sufficient, the UE should be handed off to the neighboring base station. The neighboring base station thus becomes the serving base station pursuant to the handoff.
For example, an Event A3 is defined as a measurement event in TS 36.331, V10.2.0, 2011-06 regarding such a handoff decision. In LTE, hysteresis is used to minimize “ping-ponging” (repeated handing off back and forth with regard to a pair of base stations). The UE makes a radio link quality (RLQ) measurement of the neighboring base station as well as an RLQ measurement for the serving base station. A decision to handoff the UE to the neighbor cell if:RLQNeighbor−Hysteresis>RLQServing where RLQneighbor is the radio link quality of a neighbor cell and RLQServing is the radio link quality of the serving cell. This handoff decision can be made by the serving base station based upon the RLQ measurements by the UE.
The radio link quality measurements may be based on the Reference Signal Received Power (RSRP) or the Reference Signal Received Quality (RSRQ), both of which are defined in TS 36.214, V10.1.0, 2011-03. In general, both RSRP and RSRQ measurements are conducted based on received common reference signals. The RSRP/RSRQ measurements may be simply denoted as “measurements.” Alternatively, a combination of the RSRP/RSRQ measurements and a cell search may be denoted as “measurements.”
In LTE, the user equipment makes RSRP or RSRQ measurements for the serving cell and for identified neighbor cells once every 40 ms, for example. The periodicity of the RSRP or RSRQ measurements can depend upon the particular user equipment being implemented. It may also depend on whether the user equipment is in a Discontinuous Reception (DRX) state or in a no-DRX state.
Furthermore, the user equipment needs to continuously conduct cell search for yet-unidentified cells, in addition to RSRP or RSRQ measurements for the identified neighbor cells. The user equipment conducts the search for new cells utilizing the primary synchronization signals (PSS)/secondary synchronization signals (SSS), which are transmitted once per 5 ms. Periodicity of the cell search is dependent upon the particular UE implementation and may also depend on whether the user equipment is in DRX state or in no-DRX state.
The periodicity of the measurements and associated cell search greatly affects battery power consumption in the user equipment. On the one hand, mobility is enhanced if the user equipment conducts cell search and measurements very frequently but this mobility enhancement comes at the cost of increased power consumption, which is problematic for battery-powered devices. On the other hand, power consumption may be reduced if the user equipment conducts cell search and measurements relatively infrequently but this power consumption advantage comes at the cost of reduced mobility performance. A tradeoff must thus be made to balance mobility performance and power consumption.
These same concerns are aggravated if the user equipment is to have carrier aggregation functions. In such a case, the user equipment needs to make cell search/measurements not only for intra-frequency carrier corresponding to neighbor cells, but also for secondary component carriers. In general, cell search/measurements for the inter-frequency or inter-Radio Access Technology (RAT) carrier may need more power consumption than those for the intra-frequency carrier.
Furthermore, there are other issues with cell search and measurement based on PSS/SSS/CRS. If the PSS/SSS transmitted for one cell collides with the same signals transmitted by another cell terms of time and frequency domain resources, these signals interfere with each other if they are not coded orthogonally with respect to each other. Therefore, if a user equipment needs to make measurements for multiple cells having relatively strong and colliding PSS/SSS signals, the signal-to-interference ratio (SIR) for each cell is degraded due to the resulting interference, and the cell search/measurement performance is deteriorated. This kind of problem may be denoted as “pilot pollution.” Cell search and measurements for the low SIR cells resulting from this pilot pollution require more power consumption due to the need for greater search and integration times.
It can thus be readily appreciated that the problems with cell search and measurement are exacerbated if carrier aggregation is being practiced. Accordingly, there is a need in the art for improved discovery techniques for secondary carriers.